I made scalloped potatoes today for Easter dinner and they turned black while they were cooking. I peeled and sliced them right before I put them in the casserole. My recipe said to put 1/2 the potatoes in, then 1/2 the sauce and repeat. My sauce had butter, onions, flour, light cream and gruyere cheese. I covered with foil and baked at 350 for about 1 hour then took the foil off and finished about another 20 minutes. Not only were the potatoes not cooked completely they were dark. Very disappointing in presentation but the taste was good. Any ideas as to where I went wrong? Thanks.